1. Field of the Invention
The present invention relates to a method for controlling read velocity in a disk device, and more particularly to a controlling method which adjusts read velocity to steady the data transfer rate by defect detection while the disk device read data from a disc.
2. Description of the Prior Art
Since data marks of a disc are very high density, improper manufacture, usage, and storage usually cause defects, scratches, and dust on the disc to disable data marks from being decoded in a disk device. The disk device needs to lower read velocity and correct errors many times to read data so that the data transfer rate drops to affect the read velocity and efficiency.
A prior art disc records data with mark type. For example, a known DVD (Digital versatile Disk) forms a vortex-shaped track to record data. The track is divided into 16 blocks. The structure of each block is shown in FIG. 1. Original data which want to be stored are separated into several store units. Each store unit of 2048 bytes saves in one block, and constructs the primary data of the block. In the beginning of the block, there is a header of 12 bytes in the front of the primary data. The header includes 4 bytes ID (Identification Data), 2 bytes IED (ID Error Detection Code), and 6 bytes CPM (Copyright Management Information) in order. Besides, there is 4 byte EDC (Error Detection Code) after the primary data to check the error of the header. The header, the primary data and EDC construct a 2064 byte store unit which is divided 12 sectors. Each sector has 172 bytes and is modulated by codes to record on the DVD.
In order to assure the read reliability, a 13th sector of PO (Outer Code Parity) is added after the store unit, and 10 bytes PI (Inner Code Parity) is attached after each sector. Those PO and PI are formed based on the rule of encode modulation. The header, the primary data, EDC, PO, and PI construct a block. Therefore, the block records those PO and PI codes with SYNC (Synchronization Signal) type at the same time when the primary data is recorder on the DVD. When reading data on the DVD, each store unit, PO code and PI code of 16 blocks will be read to form an ECC code (Error Check and Correction) with 182 bytes×208 sector for checking and correcting the error of data when decoding.
The prior art method for adjusting read velocity is shown in FIG. 3. When reading data on a disk, the disk device first reads each SYNC of blocks to form an ECC code, and read data to decode. Each PO and PI code checking decoded data to find error positions caused by defects, scratches and dust of the disc. An outer correction circuit (not shown) will correct and count the errors. When the counting number is bigger than a predetermined value, i.e. decoded errors are too many, the disk device lowers read velocity to precisely read data and to have enough time to handle the operation of correction errors and. On the contrary, if the counting number doesn't exceed the predetermined value, the disk device reads data with a high read velocity.
However, decoded errors generated by the disk device don't completely relate to read velocity. The prior art method for adjusting read velocity just counts all decoded errors generated by PO and PI codes without analyzing the causes of errors. So the prior art method is not able to produce the correct relation with read situation, and can't provide proper disposal against errors. Consequently, the data transfer rate of the disk device drops to affect the read efficiency. Therefore, the prior art disk device still has some problems about correction operation of ECC to resolve.